Investigators at Washington University will examine the effects of sapropterin (Kuvan) on brain and cognition in individuals with phenylketonuria (PKU) using neuropsychological and neuroimaging procedures. Sapropterin is a medication developed by BioMarin Pharmaceutical Inc. that is approved by the FDA for treatment of patients with PKU to reduce phenylalanine (Phe) levels. Patients beginning treatment with sapropterin as standard clinical care will be enrolled in the study. As a first step, patients with PKU will receive baseline neuropsychological and neuroimaging evaluations 1 day prior to beginning treatment with sapropterin. Screening for response to sapropterin will occur over 4 weeks. At the end of 4 weeks, response to sapropterin will be reviewed. Patients with a reduction of ≥ 20% in blood Phe (i.e., responders) will receive follow-up neuropsychological and neuroimaging evaluations after 6 months of treatment with sapropterin. Patients (both responders and nonresponders) will receive long-term follow-up neuropsychological and neuroimaging evaluations 3 to 5 years after initial enrollment in the study.

The focus of neuropsychological testing will be executive abilities, as these abilities are particularly susceptible to disruption in individuals with PKU. We hypothesize that improvements in these abilities will occur following treatment with sapropterin. For neuroimaging assessments, structural magnetic resonance imaging (MRI) will permit evaluation of changes in the structure and volume of the gray and white matter of the brain, whereas diffusion tensor imaging (DTI) will permit evaluation of microstructural white matter integrity.

Investigators at Washington University will examine the effects of sapropterin (Kuvan) on brain and cognition in individuals with phenylketonuria (PKU) using neuropsychological and neuroimaging procedures. Sapropterin is a medication developed by BioMarin Pharmaceutical Inc. that is approved by the FDA for treatment of patients with PKU to reduce phenylalanine (Phe) levels. Patients beginning treatment with sapropterin as standard clinical care will be enrolled. As a first step, patients with PKU who are ≥ 6 years of age will receive baseline neuropsychological and neuroimaging evaluations 1 day prior to beginning their treatment with sapropterin. Screening for response to sapropterin (20mg/kg/day) will then occur over 4 weeks as standard care for enrolled patients. At the end of 4 weeks, response to sapropterin will be reviewed. Patients with a reduction of ≥ 20% in blood Phe (i.e., responders) will receive follow-up neuropsychological and neuroimaging evaluations after 6 months of treatment with sapropterin. Patients (both responders and nonresponders) will receive long-term follow-up neuropsychological and neuroimaging evaluations 3 to 5 years after initial enrollment in the study.

A matched control group of healthy individuals without PKU will receive baseline and follow-up neuropsychological and neuroimaging evaluations for comparison purposes and to control for possible practice effects in repeated neuropsychological testing.

The focus of neuropsychological testing will be executive abilities, as these abilities are particularly susceptible to disruption in individuals with PKU. Specifically, the focus of neuropsychological assessment will be working memory, strategic processing, and inhibitory control, as our research group has shown that each of these executive abilities is impaired in individuals with PKU. (White, D. 2001 Neuropsychol.)(White, D. 2002 J. Int. Neuropsychol. Soc.)(Christ, S. 2006 Dev. Neuropsychol.) We hypothesize that improvements in these abilities will occur following treatment with sapropterin.

For neuroimaging assessments, both structural magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI; mean diffusivity and fractional anisotropy) will be used. Structural MRI will permit evaluation of changes in the structure and volume of the gray and white matter of the brain. DTI will permit evaluation of microstructural white matter integrity. Brain abnormalities have been noted in individuals with PKU, and using DTI our research group recently identified abnormalities in the integrity of white matter in early and continuously treated individuals with PKU.

The primary objectives of the proposed study are two-fold. First, we will determine whether cognition (particularly executive abilities) improves in patients with PKU who have been treated with sapropterin. Second, we will determine whether the integrity of the brain improves in patients with PKU who have been treated with sapropterin. In addition, the interrelationships between changes in cognition and brain will be examined.

Inclusion Criteria:

• Willing and able to provide informed consent or assent.
• Willing and able to comply with study procedures.
• Greater than or equal to 6 years of age.
• For phenylketonuria,intention of physician to prescribe sapropoterin.
• For phenylketonuria,phenylalanine level greater than or equal to 450μmol/L.
• For phenylketonuria, negative pregnancy test if of childbearing potential.
• For phenylketonuria, willing to use contraception if sexually active.

Exclusion Criteria:

• Pregnant, breastfeeding, or planning to become pregnant during study.
• Use of investigational product less than 30 days prior to or during study.
• Concurrent condition that could interfere with participation or safety.
• Any condition creating high risk of poor compliance with study.
• Perceived to be unreliable or unavailable for study.
• Use of L-Dopa, methotrexate, or other drugs that inhibit folate metabolism.
• For phenylketonuria, known hypersensitivity to sapropterin or excipients.

Primary care clinic for phenylketonuria. St. Louis community for control.